Field of the Invention
This invention relates to a combined GPS-inertial position locator. More specifically, it relates to a system in which inertial sensors are disposed in the antenna unit connected to a GPS receiver.
In recent years, Global Positioning System (GPS) receivers have been combined with inertial sensors to provide reliable position estimates that are updated at relatively short intervals. Specifically, GPS receivers are inherently capable of position estimates that are more accurate than those obtained from inexpensive inertial sensors. However, when disposed in a moving vehicle, GPS receivers suffer from dropouts when the signals received from the orbiting GPS satellites are attenuated by natural terrain features such as trees and hills or by man-made features such as buildings and tunnels. Moreover the GPS system provides position updates at a rate that is too slow for use in some control systems such as aircraft autopilots.
Inertial sensors, on the other hand, do not suffer from these problems, but, when implemented with inexpensive components, they provide position estimates that are much less accurate than GPS-leased estimates.
Accordingly, the two systems have been combined, with the GPS position updates being used to calibrate the inertial sensors. The sensor outputs can thus be mathematically integrated to provide accurate position estimates at a rate sufficient for use in most control systems. Moreover, they retain their calibrations for a sufficient duration to provide accurate estimates during GPS signal dropouts.
The invention is directed to an inexpensive GPS-inertial system that is suited to new installations as well as the retrofitting of GPS receivers without undue modification of existing equipment. Specifically the inertial sensors are incorporated into the housing of the GPS antenna, where their outputs are modulated onto a radio frequency carrier that has the same frequency as the GPS carrier, using the same data modulation arrangement as that used by the GPS system.
The inertial measurements can thus be sent to the receiver on the same cable as the RF signals from the antenna. Moreover, the receiver can recover the data from the sensor signals with the same arrangement that is used for data recovery from position GPS signals. Consequently, software modifications of existing GPS receivers can provide position and velocity estimates generated from the outputs of the inertial sensors.